The referenced U.S. patent application Ser. No. 297,353, filed Aug. 28, 1981, FRIESE et al, describes a spark plug which has an insulator body in which a center electrode is inserted which is made of ceramic material. It has been found that difficulties arise in the manufacture of such spark plugs since gas-tight sinter connection between a ceramic pin, the surface of which carries an electrically conductive layer or coating, and the spark plug insulator was difficult. A relatively tight fit is required. Such tight fitting, and handling of the ceramic components with tight tolerances is difficult, since ceramic materials first shaped while in deformable state, upon sintering, have the tendency to shrink. Control of such shrinkage is very difficult and, thus, manufacture of ceramic elements which fit into each other with a tight fit is difficult to achieve. While the spark plugs, as described in the aforementioned application, provided excellent results, their manufacture under mass production conditions proved difficult and resulted in a commercially excessive number of rejects.
It has previously been proposed--see U.S. Pat. No. 4,427,915, to which German Patent Disclosure Document DE-OS No. 30 38 649, corresponds--to so construct a center electrode of a spark plug that a metal-ceramic composite material is pressed in the longitudinal bore of a raw insulating body in form of a pellet, and is then finish-sintered together with the insulating body, for complete sintering of the elements. The metal-ceramic composite material contains an additive, such as a binder, which is also used as the slipping material during pressing. Typical materials are varnish or paraffin. Spark plugs which have center electrodes made of a composite metal-ceramic material require high ignition voltages and, thus, are not suitable for many standard applications.